BPICK overlap with CMIX in Zbt

[aswaterman]

These instructions perform the same function.

Zbt is not yet frozen, so the resolution is somewhat arbitrary: we can delete either instruction, then add the other instruction to both P and Zbt.

[jnk0le]

Zbt can be potentially placed into reserved FMA4 encoding riscv/riscv-bitmanip/issues/112

[chuanhua]

I prefer to keep BPICK in P and add it to Zbt (i.e., delete CMIX in Zbt).

[jnk0le]

Should the rest of Zbt land in OP-P as well?

[chuanhua]

From the perspective of keeping all Zbt instructions together in a group, I would delete BPICK in P and add CMIX from Zbt to a P sub-extension, Zbpb, created to handle overlapping issues with B extension.

[chuanhua]

Release v0.9.7 (0560ad6) adds cmix to replace bpick.